Wavelength Division Multiplexing (WDM) communication system has been developing as an optical communication system for transmitting a plurality of optical signals. In this system, for example, an Er-doped fiber amplifier (EDFA) is arranged at a predetermined place on an optical path, and a pumping laser module having a semiconductor laser device as a pumping light source is connected to the EDFA. Optical signals transmitted from a signal light source are optically amplified by a pumping laser beam emitted from the pumping laser module and introduced to the EDFA, and the optically-amplified optical signals are transmitted downstream.
Here, in order to stabilize the optical power of the pumping laser beam emitted from the laser module, the value of a current injected into the semiconductor laser device incorporated in the laser module is varied according to variation in the optical power of the signal light source.
In the case of a semiconductor laser device whose emitting wavelength is in the 1480 nm region, the gain band of the EDFA is broad, so that the above-mentioned treatment for stabilization is effective. However, in the case of a semiconductor laser device whose emitting wavelength is in the 980 nm region, the gain band of the EDFA is narrow, so that the above-mentioned treatment for stabilization cannot be taken.
Thus, when a laser module is fabricated using a semiconductor laser device whose lasing wavelength is in the 980 nm region, it is necessary to arrange that the wavelength of the pumping laser beam emitted from the fabricated laser module is specific wavelength suitable for the narrow gain band of the EDFA.
It is known that for stabilizing the emitting wavelength of a laser device, it is effective to operate the laser device with its emission end face (front facet) optically coupled to a Fiber Bragg Grating (FBG) having a predetermined reflection bandwidth. The reason is that the FBG has wavelength selection function and optical feedback function.
In this case, of the laser beam emitted from the laser device, a part within a predetermined wavelength band is reflected by the FBG and becomes return light. The return light is fed back to the laser device. By the action of the return light, the wavelength of the laser beam emitted from the laser device, hence, the wavelength of the pumping laser beam emitted from the laser module is stabilized at specific value within the reflection bandwidth of the FBG.
However, in the case of a GaAs laser device which is a representative semiconductor laser device whose emitting wavelength is in the 980 nm region, if a laser module is fabricated having the laser device optically coupled to an FBG, the optical power of an obtained pumping laser beam is unstable. Specifically, although the wavelength of the obtained pumping laser beam is within the reflection bandwidth of the FBG, the optical power of the pumping laser beam varies time-wise to a large degree. For example, only with variation of the operating state which is caused by variation of an injection current to the laser device, variation of ambient temperature or the like, the optical power of the pumping laser beam emitted from the laser module becomes unstable.
The reason is considered to be that in the case of the GaAs laser device, longitudinal modes easily become unstable, and that the optical power easily varies by several %.
Considering that it is required as a standard that variation of the optical power of a pumping laser beam emitted from a laser module should be normally 0.5% or less, above-mentioned phenomena are improper problems.